1. Field of the Invention
The present invention relates to a fine pattern forming process employed in pattern formation by a charged beam (e.g. electron beam, focus ion beam) direct writing to obtain a semiconductor device or an integrated circuit.
2. Description of the Prior Art
In production of electronic devices such as IC, LSI, and the like, pattern formation has hitherto been conducted by photolithography using ultraviolet rays. As the pattern dimensions of these devices have become finer, it is suggested to use a stepper lens of higher numerical aperture (NA), a light source of shorter wavelength, etc., but it invites a drawback of smaller focus depth. Further, electron beam lithography has come to be used as the pattern dimension of LSI device has become finer and the production of ASIC has started.
In the fine pattern formation by electron beam lithography, a positive type electron beam resist is requisite. A polymethyl methacrylate (PMMA) is known as a positive type electron beam giving the highest resolution, but has a drawback of low sensitivity.
Therefore, there have been presented, in recent years, many reports concerning the enhancement of sensitivity of positive type electron beam resists. These reports propose positive type electron beam resists of, for example, poly(butyl methacrylate), a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methyl methacrylate and isobutylene, polybutene-1-sulfone, poly(isopropenyl ketone) and fluoro polymethacrylate.
In all of these resists, in order to obtain a high sensitivity, an electron withdrawing group has been introduced into the side chain or an easily decomposable bond has been introduced into the principal chain to allow the principal chain to undergo easy scission by electron beam. However, they do not fully satisfy both of resolution and sensitivity. Further, they are not sufficiently good in dry etch resistance and heat resistance. Consequently, it is difficult to use them as a mask for dry etching and their usages are limited.
In recent years, there has also been provided a positive type electron beam resist obtained by adding a solution inhibitor to a novolac resin. In this resist, a solution inhibitor is added to a novolac resin (a main polymer) in order for the unexposed portions of the resist to be insoluble as highly as possible in an alkaline developer; meanwhile, in the exposed portions of the resist, the solution inhibitor is decomposed into lower molecules by the electron beam applied and thereby becomes soluble in the alkaline developer.
For example, an electron beam resist has been presented which is obtained by adding poly(methyl pentene sulfone) to a novolac resin. This resist is easily dry-etchable because it contains a novolac resin, and is highly sensitive because the polysulfone is decomposed. The resist, however, has poor resolution, and the unexposed portions are soluble in an alkaline developer to some extent, making it impossible to obtain a sufficient remaining film thickness and giving a large dimensional change in development. Thus, the resist is not practical.
Further, the electron beam lithography has various drawbacks such as poor dry etch resistance and heat resistance of electron beam resists, an adverse effect of proximate effect caused by forward or backward scattering of electron on pattern precision, an adverse effect of charging by incident electrons on pattern writing, and the like.
In order to improve these drawbacks, use of a multi-layer resist consisting of a pattern forming layer and a planarizing layer is very effective. FIGS. 4A to 4D are illustrations explaining a process for forming a multi-layer resist by electron beam lithography. In order to expect a reduced proximate effect, a high-molecular organic film as a bottom layer 31 is formed on a substrate 11 in a thickness of 2-3 .mu.m and then a heat treatment is effected (FIG. 4A). Thereon is formed, as an intermediate layer 32, an inorganic film of SiO.sub.2 or the like or an inorganic high-molecular film of SOG (spin on glass) or the like in a thickness of 0.2 .mu.m. Thereon is formed, as a top layer 33, an electron beam resist in a thickness of 0.5 .mu.m (FIG. 4B). Then, electron beam writing is effected, followed by development, to obtain a resist pattern 33P (FIG. 4C). Thereafter, the intermediate layer is dry-etched using the resist pattern as a mask, after which the bottom layer is dry-etched using the intermediate layer as a mask (FIG. 4D). By employing the above process using a multi-layer resist, a fine pattern can be formed at a high aspect ratio. However, in the process using a tri-layer resist, the steps are more complex; contamination is higher; dimensional change in pattern transfer is larger; thus, the process using a tri-layer resist is not practical.
As shown in FIG. 5, in the conventional process using a tri-layer resist, it occurs in some cases that a width of 0.5 .mu.m in design pattern 100 becomes thinner to about 0.3 .mu.m after the bottom layer is etched.
As mentioned above, the process using a trilayer resist is effective, but has various drawbacks such as complex steps, contamination, change in resist dimension during pattern transfer, and the like.
In electron beam writing, the adverse effect of proximate effect on pattern precision makes it necessary to form a bottom layer in a large thickness. Hence, there have been developed a silicon resist, an inorganic resist, etc., all of which function as a mask for the bottom layer and also as a resist layer. These resists include a resist containing a siloxane bond in the principal chain, a rudder type polysiloxane, a chalcogenide glass type inorganic resist, etc.; however, they are far from practical application because they have insufficient dry etch resistance, poor sensitivity and poor resolution. Further, these resists, since using an organic solvent as a developer, are large in variations of resist sensitivity and dimension, are low in process latitude, and have a fear of environmental pollution.
In order to solve these problems, the present inventors have completed an electron beam resist which can be developed with an aqueous organic alkali solution and which has high dry etch resistance, high sensitivity and high resolution, as well as a process for forming a fine pattern using said resist.